1. Field of the Invention
The present invention relates to a method of manufacturing a tire for vehicle wheels, comprising the following steps: forming a carcass structure having at least one carcass ply with terminal zones engaged with respective annular reinforcing structures which are axially spaced from one another; applying a belt structure in a position circumferentially outside the carcass structure; applying a tread band in a position circumferentially outside the belt structure and at least one pair of sidewalls in laterally opposite positions on the carcass structure.
The present invention also relates to a tire for vehicle wheels which can be obtained by means of the above-mentioned method, comprising: a carcass structure having at least one carcass ply with terminal zones engaged with respective annular reinforcing structures axially spaced from one another; a belt structure applied in a position circumferentially outside the carcass structure; a tread band applied in a position circumferentially outside the belt structure; at least one pair of sidewalls applied in laterally opposite positions on the carcass structure.
The invention also relates to a tire wheel comprising a mounting rim provided with bead seats defined by frustoconical surfaces converging towards the axis of rotation of the tire, away from the equatorial plane thereof, and a tire obtained with the method according to the invention provided with beads designed to fit precisely into the above-mentioned seats.
2. Description of the Related Art
The manufacture of tires for vehicle wheels involves the manufacturing of a carcass structure essentially composed of one or more carcass plies shaped in a substantially toroidal configuration and having its axially opposite lateral edges engaged with respective annular reinforcing structures, each of which normally comprises a circumferentially unextendable, metal, annular insert, usually called a bead core, and a filling element made of elastomeric material and combined with the bead core in a radially external position.
The carcass structure has, applied to it, in a circumferentially outer position, a belt structure comprising one or more belt layers formed in a closed ring, which are essentially made up of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies. Then, at a circumferentially outer position of the belt structure a tread band is applied, which usually consists of a strip of elastomer material of suitable thickness.
It is to point out that, to the aims of the present description, by the term xe2x80x9celastomer materialxe2x80x9d it is intended a rubber blend in its entirety, i.e. the assembly formed of at least one base polymer suitably amalgamated with reinforcing fillers and/or process additives of various types.
Ultimately, a pair of sidewalls is applied to the opposite sides of a tire being manufactured, each of said sidewalls covering a side portion of the tire comprised between a so-called shoulder region, arranged close to the corresponding side edge of the tread band, and a so-called bead arranged at the corresponding bead core.
The traditional production methods essentially envisage that the above listed tire components are first made separately from one another, to be then assembled during a step of the tire manufacture.
Production methods have also been proposed, whereby, instead of resorting to the production of semi-finished products, part or all of the carcass structure components are made directly during the tire manufacturing steps.
For example, U.S. Pat. No. 5,453,140, discloses a method and an apparatus which form a carcass ply by laying down, on a toroidal support shaped so as to match the tire, a plurality of cord sections arranged circumferentially in side by side relationship and sequentially obtained by cutting an individual cord supplied from a reel.
For the purposes of manufacturing the annular reinforcing structures, it is also known that, in the vicinity of each of the tire beads, the opposite ends of the individual cords forming a carcass ply are arranged, in an alternate sequence, in axially opposite positions with respect to an annular anchoring insert formed substantially in the manner of a circular crown and composed of coils of metal wire arranged in concentric circumferences, as can be understood from the patent EP 0,664,231 and the patent U.S. Pat. No. 5,702,548.
In the above mentioned art, all the cords forming the carcass ply or plies are however substantially arranged along the neutral axis of resistance to bending of the respective bead. Under this circumstance, the structural strength of the beads must necessarily rely on the rigidity of the filling inserts made of very hard elastomer material incorporated into the bead structure, the behaviour of which is affected by temperature changes due both to environmental factors and to stresses induced during normal operation.
The Applicant has found that important advantages can be achieved both in terms of simplification in the production processes and in terms of improvement of the behavioural features of the tire if the carcass ply or plies are made by suitably laying down strip-like sections on a rigid toroidal support, each comprising a plurality of cords parallel to each other, incorporated into an elastomer layer.
In this connection, the Applicant has already developed different manufacturing methods being the object of respective European Patent Applications.
For instance, in European Patent Application Nos. 97830731.2 (EP 0928680) and 97830733.8 (EP 0928702) a manufacturing method and a tire are respectively disclosed in which the carcass structure is obtained by making a first and a second carcass plies, each obtained by means of strip-like sections sequentially laid down in circumferential mutual side by side relationship.
Tires obtained following the method described in such patent applications have the end portions of the strip-like sections belonging to the first and second carcass plies disposed on respectively opposite sides relative to the annular reinforcing structures of the beads.
This expedient, in combination with the respectively crossed orientation of the strip-like sections belonging to one and the other plies, offers considerable advantages in terms of structural strength of the tire close to the beads and sidewalls.
In the European Patent Application No. 98830472.1(EP0976535), in the name of the Applicant as well, accomplishment of a carcass ply is proposed which is made by laying down a first and a second series of strip-like sections in an alternate sequence, the sections belonging to the first and second series terminating at respectively opposite sides relative to the bead reinforcing structures.
Thus, advantages can be achieved in terms of structural strength at the tire beads and sidewalls, even in the presence of a single carcass ply.
Usually tire beads, and in particular the annular reinforcing structures integrated thereinto, are conceived and shaped in a manner suitable for coupling with the respective circumferential seats provided on a rim with which the tire is to be associated, for the purpose of ensuring a steady connection between these two wheel components.
In greater detail, the engagement between each bead and the corresponding circumferential seat of the rim is such that the bead is constantly pushed, owing to the inflation pressure of the tire, against an abutment shoulder projecting radially away from the axis of rotation of the tire and defining the axially external edge of the rim. At least in tubeless tires, i.e. those without inner tubes, each circumferential seat for engagement of the bead has a frustoconical surfacexe2x80x94usually called a xe2x80x9cbead seatxe2x80x9dxe2x80x94having an extension converging towards the rotation axis into the vicinity of the equatorial plane of the tire. Each bead, which is pushed axially away from the equatorial plane owing to the inflation pressure, acts in an axial thrust relationship against the respective bead seat so as to ensure a perfect hermetic seal for the air contained in the tire.
Recently vehicle wheels have been proposed, in which the engagement seats of the tire beads have a frustoconical shape with a progression converging towards the axis of rotation away from the equatorial plane. An example of such a rim/tire assembly is described in the American patent U.S. Pat. No. 5,634,993.
In the embodiment proposed in this patent, the tire beads, which are shaped so as to match the corresponding seats on the rim, have annular reinforcing structures comprising usual bead cores around which the terminal zones of the carcass ply are axially folded back. The carcass structure, which is of the radial type, has overall a cross-sectional profile with a constant bending direction, the tangent of which close to the bead cores is substantially parallel to the equatorial plane.
The document WO 95/23073 describes a tire with beads which are particularly suitable for use on rims with frustoconical bead seats axially turned outwardly.
In the Applicant""s perception, the technical problem to be solved in this tire is that of modifying its structure so as to facilitate the production process thereof. In fact, in this tire, each terminal zone of the carcass ply is folded back axially from the outside towards the inside around an annular insert made of hard elastomeric material, the cross-sectional profile of which is substantially wedge shaped, with a base side parallel to the bead seat of the rim.
In an area close to the vertex of the wedge, the terminal zone of the ply passes, still in an axially inwards direction, around a bead core having a substantially circular cross-sectional profile. Soft-rubber filling elements are arranged in the areas surrounding the wedge-shaped annular insert and the bead core so that, following the tensioning produced along the extension of the carcass ply as a result of the inflation pressure, the bead core tends to be displaced axially towards the outside of the bead and consequently acts on an inclined surface of the wedge-shaped insert so as to increase the contact pressure of the bead against the bead seat of the rim.
The same Applicant has also proposed a tirexe2x80x94forming the subject of European patent application No. 98.110354.2 (EP0922592) which has beads suitable for stable anchoring onto the corresponding seats of a rim with frustoconical bead seats directed axially outwards.
In accordance with the present invention it has been found that the processes for the manufacture of tires with beads suitable for use on rims with frustoconical seats axially directed towards the outside may be significantly simplified if each bead of the tire is provided with an annular reinforcing structure having preferably a cross-sectional profile substantially in the form of an xe2x80x9cLxe2x80x9d with a radially external branch integral with the carcass ply or plies and a radially internal branch substantially parallel to the seat of the rim and designed to act with a thrust on the latter as a result of the tension transmitted to the carcass ply or plies.
The Applicant has in fact perceived that such a tire can be manufactured in a convenient manner by supplying the components thereof substantially in a direction radial to the axis of rotation and/or tangential with respect to the circumferential extension of the tire itself, substantially without axially directed movements, in accordance with the methods described in the above-mentioned European patent applications No. 97830731.2 (EP 0928680) and No. 97830733.8 (EP 0928702) as well as No. 98830472.1 in the name of the Applicant herself, hitherto not published.
In greater detail, the invention relates to a method of manufacturing a tire for vehicle wheels, characterized in that the manufacture of the carcass structure comprises the following steps: applying, on a toroidal support shaped so as to match the internal superficial extension of said tire, at least a first part of at least one carcass ply defining axially inner terminal zones of said at least one carcass ply; applying, radially superimposed with respect to an end portion of one or both said axially inner terminal zones, at least one circumferentially unextendable, annular, anchoring insert, having a cross-sectional profile with a flattened form extending axially away with respect to the corresponding inner terminal zone and an equatorial plane of the tire; applying, against each inner terminal zone, at least one stiffening element having at least one main portion with a cross-sectional profile tapering away from said axis of rotation, located substantially in an axially internal position with respect to said annular anchoring insert.
It is also preferable to carry out the stage of applying at least a second part of said at least one carcass ply defining axially outer terminal zones of said at least one carcass ply.
In a preferred embodiment, application of the second carcass ply part is performed after application of the stiffening element, so that the outer terminal zones are each superimposed on the respective stiffening element on the opposite side with respect to said inner terminal zone.
More particularly, the application of each annular anchoring insert is performed by means of winding of at least one elongated element in axially adjacent concentric turns around the toroidal support.
Preferably, an end portion of at least one of said outer terminal zones is arranged against an extended portion of the respective stiffening element, extending substantially parallel to said annular anchoring insert.
It is also advantageously envisaged carrying out the additional step of applying, radially superimposed on an end portion of at least one of said outer terminal zones, at least one circumferentially unextendable, additional, annular insert extending substantially parallel to said annular anchoring insert.
Application of the additional annular insert is conveniently carried out by means of winding at least one elongated element in axially adjacent concentric turns around the toroidal support.
In accordance with a preferred constructional solution, application of the first and/or, where present, the second part of said at least one carcass ply is performed by laying, respectively, at least one first and/or one second series of elongated sections circumferentially distributed on the toroidal support, each of said elongated sections extending in a U-shaped configuration around the cross-sectional profile of the toroidal support, so as to define two lateral portions which are mutually spaced apart in the axial direction and support said terminal zones, and a crown portion extending in a radially external position between the lateral portions.
The sections of the first series may be advantageously laid at a circumferential pitch greater than their width, the sections of the second series each being laid with its crown portion in the space in between the two adjacent sections of the first series, so as to form together wish the latter said at least one carcass ply.
In accordance with a possible constructional variant, application of the stiffening element may be carried out before application of the annular anchoring insert.
It may also be envisaged that the second part of the carcass ply is applied before application of the anchoring insert, the latter being preferably applied so as to radially superimpose an end portion of the respective outer terminal zone.
It is also preferably envisaged that, during application of the second part of said at least one carcass ply, an end portion of each outer terminal zone is arranged against an end portion of the respective inner terminal zone, extending axially away with respect to the equatorial plane.
Preferably, a step is also carried out, consisting in applying at least one circumferentially unextendable, additional, annular insert having a flattened cross-sectional profile and extending radially against an axially inner wall of the main portion of the respective stiffening element.
More particularly, each additional annular insert is applied against one of said inner terminal zones, before application of the respective stiffening element.
Application of each additional annular insert is preferably performed by means of winding of at least one elongated element in radially adjacent concentric turns around the toroidal support.
In accordance with a further possible constructional variant, application of said at least one second part of carcass ply is carried out before application of said at least one stiffening element, so that at least one of said outer terminal zones is located between the respective inner terminal zone and the respective stiffening element.
The method according to the present invention is advantageously carried out substantially without movements directed parallel to the axis of rotation of said toroidal support and aimed at axially folding back towards said equatorial plane a terminal zone of said at least one carcass ply.
The present invention also relates to a tire for vehicle wheels, characterized in that at least one of said annular reinforcing structures comprises: at least one stiffening element applied against said at least one carcass ply and having at least one main portion with a cross-sectional profile tapering away from a rotation axis of the tire; at least one circumferentially unextendable, annular, anchoring insert having a cross-sectional profile with a flattened form, extending axially away with respect to the main portion of the stiffening element.
Preferably, each annular anchoring insert extends in a direction converging towards the geometric axis of rotation of the tire away from an equatorial plane thereof.
Advantageously, the annular anchoring insert and the stiffening element are rigidly connected together, so as to define a substantially one-piece structure formed in the manner of an xe2x80x9cLxe2x80x9d.
The cross-sectional profile of each annular reinforcing structure conveniently has a geometric centre of gravity located in a position such that an axially outer end edge of said annular anchoring insert tends to be moved towards the axis of rotation of the tire following a tension produced along said at least one carcass ply as a result of the inflation pressure of the tire.
In greater detail, the cross-sectional profile of each annular reinforcing structure has a geometric centre of gravity located in a position axially external with respect to said stiffening element and axially internal with respect to an axially outer end edge of said annular anchoring insert.
Preferably, said at least one carcass ply has a first and a second part defining respectively axially inner terminal zones and axially outer terminal zones.
In a preferred constructional solution, said stiffening element is axially located between the respective axially inner terminal zone and the respective axially outer terminal zone of said at least one carcass ply.
Alternatively, the stiffening element may be applied in a position axially outside both the inner end and outer end of the carcass ply.
The annular anchoring insert may be advantageously applied, preferably in a radially external position, against an end portion of said inner terminal zone, extending axially away with respect to an equatorial plane of the tire.
The stiffening element advantageously comprises at least one annular body made of elastomeric material preferably having a hardness greater than 48xc2x0 Shore D.
Preferably, said annular anchoring insert is arranged substantially in the vicinity of a circumferentially internal edge of the main portion of the stiffening element.
Said stiffening element may have, in a radially internal position, an extended portion extending substantially parallel to said annular anchoring insert.
The presence of at least one additional annular insert extending parallel to said annular anchoring insert is also preferably envisaged.
This additional annular insert may be applied, preferably in a radially external position, against an end portion of said outer terminal zone, extending axially away with respect to an equatorial plane of the tire.
In a preferred constructional solution, said at least one carcass ply comprises a first and/or a second series of elongated sections circumferentially distributed, preferably in an alternate sequence, around said axis of rotation and each extending in a U-shaped configuration around the cross-sectional profile of the carcass structure so as to define two lateral portions mutually spaced in the axial direction, and a crown portion extending in a radially external position between the lateral portions.
Axially inner terminal zones and axially outer terminal zones of said carcass ply are respectively defined on the lateral portions of the sections of the first and the second series.
The annular anchoring insert may be conveniently applied in a radially external position against an end portion of said outer terminal zone, extending axially away with respect to an equatorial plane of the tire.
The end portion of the outer terminal zone may be applied so as to be radially superimposed against an end portion of the inner terminal zone.
It is also possible to envisage at least one circumferentially unextendable, additional, annular insert having a cross-sectional profile extending radially against an axially inner wall of the main portion of said stiffening element.
The invention also relates to a tire wheel characterized in that it comprises a mounting rim provided with bead seats defined by frustoconical surfaces converging towards the axis of rotation of the tire, away from the equatorial plane thereof, and a tire obtained with the method according to the invention and provided with beads designed to fit precisely into the above-mentioned seats.
The above-mentioned rim is characterized by the presence of axially internal bead-unseating safety humps with a minimum or even zero height, preferably in combination with the symmetry of its radially external profile.
This aspect of the invention is based on the recognition of the technical problem associated with passing-over of the above-mentioned hump by the bead depending on the structure of the bead itself.
It has been found that passing-over of the hump by the bead can be controlled more easily when there is a multiple-turn bead core lying in a plane substantially perpendicular to the axis of rotation and/or with turns distributed over a frustoconical surface substantially parallel to the surface of the bead seat.
It has also been found that, as a result of the use of the above-mentioned multiple-turn bead cores, it is possible to reduce significantly the depth of the well located between said bead seats on the mounting rim.